1. Field of the Invention
This invention relates to measuring ac electrical current, and in particular to a method and measurement system which extends the range of measurements made with a current transformer which saturates during operation.
2. Background Information
Current transformers (CTs) are commonly used for sensing ac electrical currents. For instance they are often used in load control and protection devices such as contactors; motor starters and controllers; circuit breakers; monitors and analyzers; and with electrical distribution systems. In many such applications the load current can have a very wide dynamic range. Unfortunately, the magnetic materials commonly available for the cores of the current transformers limit the dynamic range of the sensing device. Peak flux density is a limiting factor at the upper end of the dynamic range, while core loss/declining permeability is a limit at the lower end. For a given core material and required accuracy, these parameters limit the operating range of the current transformer. While the dynamic range could be extended by increasing the volume of the core material and/or the turns of the secondary winding, these solutions increase the size of the CT which is often critical, and also increase the cost which can be unacceptable in low end products. As many of these products are multiphase, the effects of these factors are compounded.
There is a need therefore for an improved current transformer system for current sensing.
There is a particular need for a current transformer sensing system which provides increased dynamic range for the same size device or the same dynamic range but with a smaller device.